The objective of this research is to characterize the initial steps in influenza virus replication and to identify the viral and cellular components that are needed to attach the virion to host cells and to move the viral transcription complex into the cytoplasm. We want to know in detail how the influenza viruses initiate infection and how the initial steps in this process are involved in te adaptation of these viruses to growth in new hosts. We will characterize two host dependent plaque type mutants of influenza virus which have different hemagglutinins. We will determine how the hemagglutinin proteins of the two variants differ in composition and in biological properties. Our aim is to determine how changes in the structure of the hemagglutinin affect interactions of this viral glycoprotein. We will then use these host dependent hemagglutinin variants and recombinant viruses to delineate the virus structural requirements for each early step in the infectious process. In addition, we will determine the cellular factors involved by using fully permissive cells and virus resistant cells to independently measure adsorption, internalization, and introduction of the viral transcription complex into the cytoplasm. In view of our inability to control influenza by conventional immunological measures or to develop antiviral agents which block intracellular steps in viral replication, it is important that we understand the initial events in the infectious process. A thorough understanding of this process could provide a rational approach to the control of these viruses. Secondly, we now have reason to expect that a thorough characterization of those initial steps will help us to understand how viral strains from one animal species establish infection and acquire the ability to grow well in another animal species. Since one of the abiding problems in the control of human influenza is the introduction of influenza viruses from animal reservoirs into the human population, an understanding of the mechanisms involved in these interspecies transfers may also be critical to the control of this disease.